Chunk server: write append checksum optimization: use incremental checksum computation for the network checksum verification, store computed checksum vector, and use it with chunk write, instead of recomputing checksum to write. This is possible because chunk append position is known at the append "begin" time, and the checksum computation optimization has virtually no effect on the append state machine. Restore appender's checksum vector in the case if the request checksum verification fails, to minimize complexity as request is effectively ignored in such case.